1. Field of the Invention
This invention relates, in general, to electrical inductive apparatus and, more specifically, to vaporization-cooled electrical inductive apparatus.
2. Description of the Prior Art
Vaporization cooling systems have been proposed for electrical inductive apparatus, such as transformers, reactors and the like, which utilize a two-phase dielectric fluid having a boiling point within the normal operating temperature range of the electrical inductive apparatus. The dielectric fluid is supplied to the electrical inductive apparatus in its liquid state, whereon it evaporates as it contacts the heat producing members and removes heat in quantities equal to the latent heat of vaporization of the dielectric fluid. The resulting vapors are then condensed and reapplied to the heat producing elements in a continuous cycle. In addition to providing cooling, the vaporization of such a dielectric fluid also provides the necessary electrical insulation between the electrical elements in its vapor phase at the nominal operating temperature and pressure of the electrical inductive apparatus. Since the insulating properties of the vaporized dielectric fluid are directly proportional to the pressure existing within the enclosure surrounding the electrical apparatus, it is common to add a second dielectric fluid, typically a non-condensable gas, such as sulfur hexafluoride (SF.sub.6), to provide adequate electrical insulation when the apparatus is initially energized or operating at very light loads.
It is known that vaporization-cooled electrical inductive apparatus, such as transformers, have a relatively short thermal time constant compared to conventional oil-filled transformers which results in a rapid increase in the temperature and pressure within the casing during transient overload conditions. In an oil-filled unit, excessive temperature normally results only in the loss of insulation life. In a vaporization-cooled electrical apparatus, an overload results in a large pressure increase which, if permitted to continue, would require compliance with expensive pressure vessel construction codes to prevent rupture of the transformer tank.
A typical prior art method of increasing the overload capacity with vaporization-cooled transformer has been to add additional cooling surfaces to the cooling system of the transformer. However, a large increase in cooling capacity is required to provide adequate overload capacity due to the exponential increase in heat generated by the transformer I.sup.2 R losses. For example, a 1,000 KVA transformer has a 100% load loss of 11.5 KW and a 200% load loss of 37.5 KW. The additional cooling capacity required to dissipate the additional heat loss caused by overload conditions is not only expensive but also results in an objectionable increase in the overall size of the electrical apparatus.
Thus, it would be desirable to provide a vaporization-cooled electrical apparatus which has an improved overload capacity. It would also be desirable to provide the improved overload capacity for a vaporization-cooled electrical inductive apparatus by an inexpensive method and, further, in such a way that the overall size of the electrical apparatus is not significantly increased.